1. The Field of the Invention
This invention relates generally to line isolation of electronic circuitry. More particularly, this invention relates to isolation circuitry for use in interfacing user equipment with telephonic or other networks.
2. Present State of the Art
Traditional custom equipment that interfaces with custom networks or systems may employ a variety of interface standards. However, interface devices such as end user equipment that interfaces with established networks, such as the telephone network, are required to employ an established interfacing standard. One such standard requires end user equipment to provide electrical isolation between user equipment and the telephone network. Such isolation requirements stem from regulatory agencies such as the Federal Communications Commission and other counterpart regulatory agencies in other countries. A fundamental purpose for such regulations is to provide safety and protection to the established telephone network. Such protection prevents end user equipment from accidentally injecting or accepting into the protecting network and the user/consumer telephone network extraneous or hazardous signals.
While specific regulations and requirements may vary from country to country, established circuits which facilitate the isolation requirements of most regulatory agencies have become commonplace. One such circuit component is a Direct Access Arrangement (DAA). As shown in FIG. 1, DAA 112 provides the most proximate interface of user equipment with a telephone network 104. As an exemplary configuration of user equipment, FIG. 1 illustrates a host 100 which may take the form of a personal computer or other interfacing equipment. As many types of end user equipment generate digital data, a modem 102 is required to modulate and demodulate digitized data into an analog format capable of propagation through telephone network 104. An exemplary modem 102 is further comprised of a host interface 106 generally taking the form of an ASIC integrated circuit or other programmable or discrete circuitry for compatible exchange of data with host 100. Most modern digital signal processors (DSP) such as DSP 108 provide, among other things, the functionality required for modulating and demodulating digital data. A CODEC 110 provides a transformation between digital data and analog data for data transmitted from host 100 and translation of analog data to digital data for data dispatched by telephone network 104 to host 100. The functionality and composition of host interface 106, DSP 108, and CODEC 110 are generally known in the art and need not be further discussed.
As data generated by host 100 and destined for telephone network 104 must exit the end user equipment and travel on the telephone network, the illustrative modem 102 must incorporate the isolation requirements as dictated by regulatory agencies. In the present illustration of FIG. 1, DAA 112 provides the isolation requirements for protecting telephone network 104 from extraneous and injurious signals. To prevent such extraneous and injurious signals from passing from host 100 to telephone network or to endanger the uswer 104, an isolation boundary 202 is established wherein the signals generated by user equipment 100 must be coupled across isolation boundary 202 to telephone network 104.
In FIG. 2, a traditional DAA 200 is depicted as implementing the required isolation boundary 202. As described above, isolation boundary 202 forms a partition between a user side connected to a host, such as a personal computer, and a line-side connected to the network, such as a telephone network. Traditional DAA implementations have incorporated on the user side a user side signal interface 204 to provide access to transmit and receive signals coupled to a hybrid circuit 216 which, due to the combining of transmit and receive signals on the line-side, requires that the transmit signal be cancelled or removed from the receive signal.
Isolation boundary 202 is then established across a transformer 208 which provides the required coupling between the user side and the line-side. Because of the inherent nature in transformer coupling, the user side and the line-side may float with respect to a potential across isolation boundary 202. Traditional implementations of the line-side circuitry in DAA 200 include bypass capacitors 210, a hold circuit 212, and a diode bridge 214 for generating tip and ring signals 206 characteristic of a telephone network. Hold circuit 212, among other things, incorporates an off hook switch for providing the DC current path necessary to notify a telephone network of an off-hook condition and a ring detector for notifying a user of an incoming communication. Other isolation boundary implementations include capacitively coupling the analog transmit and receive signals across the isolation barrier by providing a pair or pairs of capacitors that differentially couple analog transmit and receive signals from a user side across the isolation barrier to a line-side.
While modem isolation implementations of the past have generally been transformer based, there have also been other methods such as linear optical solutions that use an active region of an optical isolator for transfer of analog signals across the isolation boundary. While transformer designs were feasible for physically larger designs, transformer designs incorporated into smaller integrated products compromise the linearity and also generate insertion loss and a degraded frequency response as transformers are reduced in physical size. Linear optical approaches are also feasible, but require gain adjustments and tuning for variations in gains associated with optical isolator production runs. Furthermore, linear optical isolators are more expensive than standard optocouplers.
Thus, what is needed is a method and system for providing an isolation boundary between a user side and a line-side for equipments interfacing with a defined network, such as a telephone network, that require an isolation boundary. Furthermore, what is desired is a method and system for providing an isolation boundary utilizing isolation components that lend themselves to economical integrated designs without subjecting themselves to the degraded affects associated with miniaturized magnetic components.